In seven consecutive cases of transposition of the great arteries balloon atrial septostomy was performed under the sole echocardiographic guidance. The catheter, introduced into either the saphenous, femoral or umbilical vein, was advanced into the inferior vena cava, then into the right atrium, and, through the patent foramen ovale, into the left atrium. The correct location of the tip of the catheter within the left atrial body was easily identified by two-dimensional echocardiography, with a four chamber view from the subcostal approach. Under echocardiographic control alone the inflated balloon was then suddenly withdrawn into the right atrium according to the Rashkind's technique. The procedure was repeated 3 to 5 times. The balloon was inflated with 2 to 3.5 ml of saline solution. The septostomy was deemed to be adequate when the echocardiographic images revealed an atrial septal defect at least 5 mm in size and an increased flapping motion of the inferior rim of the atrial septum. With this technique the risks of complications due to an incorrect location of the catheter are minimized and the outcome of the procedure can be immediately evaluated by measuring the size of the atrial septal defect.